1. Field of the Invention
This invention relates a steering system for a vehicle, and more particularly to a steering system for a vehicle in which the steering ratio, that is, the ratio of the turning angle of the steering wheel to the turning angle of the dirigible wheels, is variable.
2. Description of the Prior Art
As is well known, in the steering system of a vehicle, rotary motion of the steering wheel is converted into lateral sliding motion of tie rods by way of a steering gear such as a rack-and-pinion mechanism and right and left dirigible wheels connected to the ends of the tie rods are turned in response to the sliding motion of the tie rods. Conventionally, the steering ratio, i.e., the ratio of the turning angle of the steering wheel to the turning angle of the dirigible wheels (which are generally the front wheels) has been fixed. However, it is preferred in view of running stability that the steering ratio be increased to reduce the turning angle of the front wheels for a given turning angle of the steering wheel during high speed travel. On the other hand, when the vehicle speed is low, it is preferred that the steering ratio be reduced to increase the turning angle of the front wheels for a given turning angle of the steering wheel, thereby improving response of the vehicle to operation of the steering wheel and facilitating control of the vehicle position.
Thus, there has been proposed in Japanese Unexamined Patent Publication Nos. 58(1983)-224852 and 54(1979)-100031 a speed-responsive steering system in which the steering ratio is changed according to the vehicle speed. In the steering system, there is provided between the steering wheel and the steering column a transmission mechanism comprising a pair of variable pitch pulleys and a V-belt passed around the pulleys, the pitch diameter of the driven pulley being arranged to be increased by a stepping motor as the vehicle speed increases.
The speed responsive steering system has to be arranged such that the turning angle of the front wheels is changed when the vehicle speed changes with the turning angle of the steering wheel fixed. Otherwise, the front wheels cannot be returned to the neutral position when the steering wheel is returned to the neutral position. However since the driving speed of the stepping motor, i.e., the control speed of the steering ratio, is fixed irrespective of the vehicle speed, and since failure of the controller for controlling the stepping motor is generally detected a predetermined time after the occurrence of the failure, the turning angle of the front wheels is changed by the same amount irrespective of the vehicle speed by the time the failure is detected. Accordingly, the turning angle of the front wheels can be changed by amount exceeding a proper range when the vehicle speed is high.
When the driving speed of the stepping motor is reduced to obviate such danger at high speed, the response to operation of the steering wheel will be adversely affected.